Suddenly, and quite unexpectedly, this rich slice of investigative journalism turns into something still more substantial. We've got the headline points easily enough. We know that the Kean/Hamilton Commission, examining America's response to 9/11, was supposedly nobbled by an executive director in league with the White House. We know that Professor Philip Zelikow, the nobbler in question, had actually been part of the Republican transition team and a close collaborator of Condoleezza Rice. Worse, we know he's risen to bureaucratic glory since as State Department counsellor at Ms Rice's right hand. 'Independent' congressional inquiries don't come any dodgier than this.

But that, as Shenon's account of the committee's turbulent history races along, also comes to seem a pretty constrained sensation. What's much more fascinating is his dissection of the politicking that dogged this commission and which will surely dog any heirs and successors when they try to take on associated debacles like Iraq. Indeed, from the Warren Commission (which investigated the killing of President Kennedy) to pallid British parallels, the lessons are eerily similar.

First, in Capitol Hill terms, choose your chairman. Dr Kissinger, we presume? But he ducks out because his company has too many Saudi clients, and in comes Tom Kean, a courtly Republican former New Jersey governor turned university vice-chancellor. And a balancing Democratic deputy chairman? George Mitchell, of Belfast fame? But he's too busy. Enter ex-congressman Lee Hamilton from Indiana, a mild, consensual chap. Already the shape of things to come is predestined. If the big names won't play, the lesser names had best stick together as co-chairmen and make as few waves as possible.

They pick Zelikow as staff supremo because, though choleric, he seems strong and persuasive. They are warned of their mistake. They press on. The Democrats on the commission reckon that the administration will be unhelpful going on hostile and want to subpoena their evidence from day one. The co-chairs don't agree. Delays and frustration ensue. Neither the funding nor the time is sufficient. Even getting security clearance for commission staff takes months. Writing any sort of report, let alone a wise one, becomes a formidable challenge.

At which point, frankly, the alleged importance of Philip Zelikow begins to recede a little. He clearly doesn't want Rice involved. He clearly thinks that the CIA, led by a charming director with a shocking memory, should take most stick. Others, Shenon included, are inclined to dump more opprobrium on the FBI's doorstep. They knew about some of the skyjackers. They had them on a plate. But they were too lumpenly bureaucratic, too administratively arthritic, to get such news from bottom to top. Their old director didn't even have a computer in his office.

But at least his replacement, as acting chief, co-operates fully and effusively, unlike George Tenet at the CIA. And thus the sheer pressure of a publication deadline turns the commission's fire away from the bureau. It's the agency that loses autonomy in the final report (as Messrs Bush, Rove and Rice duck for cover). Did Kean/Hamilton do a decent job? Not too bad, in the circumstances, but the huge virtue of Philip Shenon's study is to lay out those circumstances in full. Discover a problem, call for a commission - and this is probably what you get. This, also, may be as good as it gets.

We're invited to side with Richard Clarke, the director of counterterrorism at the National Security Council, who Zelikow in transitional mode tried to demote. We're supposed to fit that into a pattern of Bush culpability and (proven) Cheney lies. And it's absolutely true that the Bush team, full of evasions and blanknesses, let 9/11 happen on their watch and could conceivably have prevented it. The difficulty (for us, as for Shenon, a fine, fair-minded New York Times reporter) is being quite certain how to find your own way through this Parkinsonian maze in which everybody, Clarke included, plays an expected role.

His job is combating terrorism. He warns his masters about terrorist threats day after day. But he would, wouldn't he? Without such warnings, he wouldn't exist. And somewhere in the melee of messages from every delegated side, you glimpse the essential failure of 9/11. Not that nobody told Bush and Rice what might happen, but that everyone set them different priorities, some muttered, some shouted, and that they chose the wrong basket.

Osama bin Laden is the greatest creator of jobs worldwide in the 21st century - wander into any airport and see - but the jobsworths couldn't catch him and haven't caught him yet. The might of the CIA, FBI and National Security Agency equalled incompetence. So, on this evidence, did al-Qaeda's blunderings. Human and organisational weakness ties hunters and hunted together. It's Shenon's great virtue that The Commission is a masters' thesis in human frailty, an open door to deeper understanding of threats so complex that one man and one answer can never be enough.